The hypothesis that plasma renin is a major risk factor in the development of cardiovascular complications of hypertension was evaluated in a model animal, the spontaneously hypertensive rat (SHR). Plasma renin levels and pathological findings were studied in stroke-prone and -resistant strains of SHR at various ages. The stroke-prone strain showed significantly higher plasma renin levels at and after seven months of age, while the stroke-resistant strain showed no significant differences from the control at any age. High plasma renin levels were always associated with evident vascular complications in the kidney and the brain, indicating underlying angionecrosis and malignant transformation of hypertension. However, no evidence indicated that a higher plasma renin preceded the development of vascular lesions. Thus the high plasma renin would be a result rather than the cause of hypertensive vascular lesions.